This application pertains to the art of illumination systems used in connection with automated visual inspection systems and will be described with reference thereto. However, it will be appreciated that the invention has broader applications, such as in the provision of an extremely reliable and uniform lighting system for any application requiring controlled illumination.
Machine vision continues to obtain increasing significance in industry to aid in robotic assembly systems as well as inspection systems for product sorting or quality control. Such machine vision systems are comprised generally of a lighting system to illuminate a specimen and a camera for capturing light reflected therefrom. A digitized image is formed from the light received by the camera.
More recently, implementations of configurable, solid-state lighting arrays and machine vision systems have improved significantly overall performance levels and quality in such systems. See, for example, U.S. Pat. No. 4,882,498 to Cochran et al., commonly owned by the assignee hereof and incorporated herein by reference.
While initial techniques for forming solid-state lighting arrays provided significant improvement over earlier lighting systems, they nonetheless provided some limitations in obtainable total light output intensity, as well as being expensive to fabricate. These concerns are particularly significant in applications employing large lighting arrays, such as required for inspecting materials provided in a continuous web format, such as textiles, films, paper, metals, and the like.
Configurable solid-state lighting arrays are presently fabricated using individually packaged LED components. In such a construction, an individual light emitting p-n junction chip is typically encapsulated in a transparent epoxy. The epoxy acts to mechanically support the sensitive diode. Additionally, the epoxy capsulation is often molded into a spherical shape, thus giving it some lensing action. The lens-like characteristics of the epoxy encapsulation effectively concentrates the broad angular distribution of light emitted by a diode junction into a limited cone angle.
Such LED construction is often useful in applications in which the device is used as a panel or circuit board indicator. However, in machine vision applications wherein it is desirable to generate a uniform illumination pattern over a broad spatial field, the tendency of a lensed LED to generate illumination "hot spots" is deleterious.
Earlier attempts to address the hot spot problem have employed such means as diffusers disposed between LEDs and a target, or with an increase of a distance between the LED light source and the target.
Yet another draw back inherent to conventionally fabricated solid-state lighting arrays is the relatively large physical space requirement for epoxy packaging. A typical light emitting surface of a p-n junction is approximately 0.010 inches square. This small junction is usually encapsulated in a package with a diameter ranging from 0.10 inches to 0.25 inches. Thus, the ability to pack individual LEDs together into an array is constrained to a large degree by the packaging of the individual LED devices themselves.
Yet another disadvantage of illumination sources employing individually packaged LEDs is provided by virtue of the fact that the epoxy material in which they are encapsulated is a poor heat conductor. An important factor which limits the amount of light which may be emitted from an LED is the surface temperature of the associated emitting p-n junction. As surface temperature increases, the current-to-light-conversion efficiency of the device decreases correspondingly. Additionally, as the drive current of a device is increased, the power dissipated by the LED in the form of heat also increases. This tends to raise the surface temperature of the p-n junction. Thus, conventional LEDs are self-limited in the amount of light which they can generate.
The subject invention overcomes the above problems, and others, providing a dense array of solid-state light emitting diodes capable of providing an extremely high light output.